Heterojunction bipolar transistors (HBTs) offer much higher speed of operation than the more prevalent metal-oxide-semiconductor field-effect transistors (MOSFETs) or even conventional homojunction bipolar transistors, e.g., npn or pnp silicon transistors. In some applications where a high degree of linearity is required, HBTs are the alternative technologies of metal semiconductor field effect transistors (MESFETs) and high electron mobility transistors (HEMTs). The use of different materials of differing bandgaps for the collector, base, and emitter provides design flexibility.
The most common HBT technology is based on the GaAs/GaAlAs material family, several features of which are explained by Asbeck in High-Speed Semiconductor Devices, ed. Sze (Wiley, 1990), pp. 335-338, 358-366, and 370-384.
Asbeck describes one such GaAs HBT, ibid., p. 376, as shown in the cross-sectional view of FIG. 1. Over a semi-insulating GaAs substrate 10 is formed an n.sup.+ -type collector contact layer 12, to which a collector contact C is applied. An n.sup.- -type collector layer 14 and a p.sup.+ -type base layer 16 of GaAs or GaAlAs is grown thereover. A small n.sup.- -type emitter 18 is formed over the base layer 16. An emitter contact E is applied to the emitter 18, and a ring or multiple base contacts B are applied to the base layer 16.
A disadvantage of the structure described thus far is that the active area is determined by the area of the emitter 18, but the parasitic capacitance is largely determined by the much larger area of the interface between the base and collector layers 16, 14. One method for reducing the base-collector capacitance implants protons into an annular area 20 of the collector layer 14 surrounding the emitter 18 so as to render the implanted area 20 electrically insulating. Asbeck reported a reduction of the capacitance by a factor of two by using proton implantation.
Another HBT technology is based on the InP/InGaAs material family, as is explained by Asbeck, ibid., pp. 384-388. HBTs composed of this material family offer higher speed and can be integrated with opto-electronic devices that are sensitive in the 1.3 and 1.5 .mu.m optical bands, which have become so important for fiber optic communications. However, InP-based heterojunction bipolar transistors similarly suffer from high base-collector capacitance, and proton implantation has proved ineffective in rendering InP to be insulating or semi-insulating.
Miyamoto et al. have addressed this problem in InP-based HBTs in "Reduction of Base-Collector Capacitance by Undercutting the Collector and Subcollector in GaInAsInP DHBT's," IEEE Electron Device Letters, vol. 17, March 1996, pp. 97-99. By use of a selective etchant, they substantially etched the collector layer under the base layer so as to undercut the edges of the base layer. To provide some mechanical integrity, they then backfilled with polyimide. The reduced size of the collector, together with the lower dielectric constant of the polyimide, reduced the base-collector capacitance. They also reported a reduction of capacitance by about a factor of two. However, the robustness of this structure remains in question.